Identification of packet traffic transmitted by various devices operated in multiple overlapped frequency bands in packet-based OFDM systems

ABSTRACT

Techniques for header encoding include encoding a plurality of bits using a forward error correction code, generating an FEC codeword comprising a plurality of encoded bits, and concatenating a first copy of the FEC codeword with a second copy of the FEC codeword, wherein the concatenating comprises cyclically shifting by two bits the second concatenated copy of the FEC codeword relative to the first concatenated copy of the FEC codeword, wherein the encoded bits of the first and second copies of the FEC codewords are modulated on at least one OFDM symbol. techniques for header decoding include receiving a plurality of encoded bits comprising at least two concatenated copies of an FEC codeword, decoding a first copy of the FEC codeword to generate a first plurality of decoded bits, and decoding a second copy of the FEC codeword to generate a second plurality of decoded bits.

RELATED APPLICATION DATA

This application is a Continuation of U.S. patent application Ser. No.16/849,358, filed Apr. 15, 2020, now U.S. Pat. No. 11,032,401, which isa Continuation of U.S. application Ser. No. 16/145,971, filed Sep. 28,2018, now U.S. Pat. No. 10,637,971, which is a Continuation of U.S.patent application Ser. No. 15/468,903, filed Mar. 24, 2017, now U.S.Pat. No. 10,091,337, which is a Continuation of U.S. patent applicationSer. No. 15/148,216, filed May 6, 2016, now U.S. Pat. No. 9,609,092,which is a Continuation of U.S. patent application Ser. No. 12/783,903,filed May 20, 2010, now U.S. Pat. No. 9,363,121, which is a Continuationof U.S. patent Ser. No. 12/743,876, filed Dec. 28, 2010, now U.S. Pat.No. 8,537,796, which is a national stage application under 35 U.S.C. 371of PCT Application No. PCT/US2009/054915, having an international filingdate of Aug. 25, 2009, which is designated the United States, which PCTapplication claims the benefit of and priority under 35 U.S.C. § 119(e)to U.S. Patent Application Nos. 61/091,621, filed Aug. 25, 2008,entitled “Methods to Identify Packet Traffic Transmitted by VariousDevices Operated in Multiple Overlapped Frequency Bands in Packet-BasedOFDM Systems,” and 61/196,068, filed Oct. 14, 2008, entitled “Methods toIdentify Packet Traffic Transmitted by Various Devices Operated inMultiple Overlapped Frequency Bands and Packet-Based OFDM Systems,” allof which are incorporated herein by reference in their entirety.

BACKGROUND Field of the Invention

Exemplary aspects of the invention relate to communications systems.More specifically, exemplary aspects of the invention relate tocommunications systems where information is exchanged using packet-basedtransmissions based on Orthogonal Frequency Division Multiplexing(OFDM). More specifically, exemplary aspects of the invention relate toidentifying packet traffic in order to support interoperability amongdevices operated in multiple frequency bands where packet-basedtransmission based on orthogonal frequency domain multiplexing is used.

Considering multi-user communication environments where two or moreusers communicate with one another over a shared channel, e.g., a singlefrequency band, using packet-based transmission based on OFDM, a packetis usually formed by a preamble, a header, and a payload, andtransmitted using time-sharing or contention-based media access methods.Examples of such systems includes IEEE 802.11 (Wireless LAN) and IEEE802.16 (WiMAX).

OFDM, also referred to as Discrete MultiTone (DMT) or multicarriercommunications, divides the transmission frequency band into multiplesubcarriers, also referred to as tones or subchannels, with eachsubcarrier individually modulating a bit or a collection of bits.

SUMMARY

As used herein, the terms transmitter, transmitting transceiver andtransmitting modem are used interchangeably, similarly, the termsreceiver, receiving transceiver and receiving modem are usedinterchangeably as well as the terms modem and transceiver being usedinterchangeably.

Additionally, as used herein, the following terms are defined:

Domain: A network comprising all nodes which can communicate andinterfere with each other directly at the physical layer.

Domain Master (DM): A node of a domain managing (coordinating) all othernodes of the same domain (e.g., assigning bandwidth, resources andmanage priorities).

Profile: A band plan which specifies OFDM modulation parameters such asthe range of frequency, sub-carrier spacing, cyclic prefix, etc. Theremust be one or more overlapped regions between two different profiles.

High-profile device: A device operated using a profile which has ahigher bandwidth relative to low-profile.

Low-profile device: A device operated using a profile which has a lowerbandwidth relative to high-profile.

Fsc: Sub-carrier spacing.

N: Maximum number of sub-carriers in a profile of the bandwidth B(B=N×Fsc).

Fus: Frequency up-shift used in the definition of IDFT for basebandtransmission.

Fuc: Frequency up-conversion used in the definition of IDFT for passbandor Radio-Frequency (RF) transmission.

Fc: Center frequency used in the definition of IDFT for passband or RFtransmission.

Fc=Fuc+Fus.

Exemplary aspects of the invention are directed toward identification ofpacket traffic.

Additional aspects of the invention are directed toward identificationof packet traffic transmitted by various devices operated in a multipleoverlapped frequency bands in packet-based OFDM environments.

Additional aspects of the invention are directed toward a domain mastermanaging traffic amongst a plurality of devices in a domain, with alldevices in a domain capable of identifying all traffic passingtherethrough.

Even further aspects of the invention relate to accommodating multipleprofiles within a single domain.

Additional aspects of the invention are also directed toward injectinginformation regarding profile (band plan) into a preamble so that areceiver can identify what profile the transmitter is operating on aspart of a preamble detection process.

Even further aspects of the invention relate to bypassing a headerdecoding process.

Additional aspects of the invention relate to having common sub-carrierindexing for all profiles so that a high-profile device can decode aportion of sub-carriers of a low-profile device, and a low-profiledevice can decode a portion of sub-carriers of a high-profile device.

Additional aspects relate to enhancing the decodability of a header viasub-carrier redistribution.

Additional aspects of the invention relate to distribution of headerinformation between preamble and non-preamble tones.

Aspects of the invention also relate to making a number of sub-carrierson which a single set of header information bits are mapped, and theratio of the number of non-preamble tones over the number of preambletones, be co-prime or even-odd pair.

Even further aspects of the invention relate to circular shifting ofsub-carriers within each header block, for example, by changing tonemapping as a copy of the header block is repeated over the entirebandwidth.

These and other features and advantages of this invention are describedin, or are apparent from, the following detail description of theexemplary embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The exemplary embodiments of the invention will be described in detail,with reference to the following figures, wherein:

FIG. 1 illustrates an exemplary domain according to this invention;

FIG. 2 illustrates an exemplary embodiment of a domain where multipleprofiles are operating according to this invention;

FIG. 3 illustrates an exemplary set of passband and baseband profilesaccording to this invention;

FIG. 4 illustrates an exemplary two-section preamble according to thisinvention;

FIG. 5 illustrates an exemplary portion of a packet according to thisinvention;

FIG. 6 illustrates an exemplary preamble generation system according tothis invention;

FIG. 7 illustrates a flowchart of an exemplary method for preambleassembly according to this invention;

FIG. 8 illustrates a flowchart of an exemplary method for profiledetection according to this invention;

FIG. 9 illustrates a flowchart of an exemplary method for profileselection for packet transmission according to this invention;

FIG. 10 is a flowchart illustrating an exemplary method for packetreception according to this invention;

FIG. 11 illustrates examples of sub-carrier indexing for differentbaseband and passband profiles based on an IDFT definition;

FIG. 12 illustrates examples of alternate sub-carrier indexing fordifferent baseband and passband profiles according to this invention;

FIG. 13 illustrates another example of sub-carrier indexing fordifferent passband profiles according to this invention;

FIG. 14 illustrates an exemplary transceiver according to thisinvention;

FIG. 15 is a flowchart illustrating an exemplary header identificationmethod according to this invention;

FIG. 16 is a flowchart illustrating an exemplary header identificationand demodulation method according to this invention;

FIG. 17 is a flowchart illustrating another exemplary headeridentification method according to this invention;

FIG. 18 is a flowchart illustrating another exemplary headeridentification and demodulation method according to this invention;

FIG. 19 illustrates examples of header repetition schemes used inbaseband and passband profiles;

FIG. 20 illustrates an exemplary header repetition scheme;

FIG. 21 illustrates an exemplary header repetition scheme with oddheader information block size;

FIG. 22 illustrates an exemplary header repetition scheme utilizing toneshifting;

FIG. 23 illustrates another exemplary communication system according tothis invention;

FIG. 24 is a flowchart illustrating an exemplary method for enhancedheader techniques according to this invention;

FIG. 25 is a flowchart illustrating another exemplary method forenhanced header techniques according to this invention;

FIGS. 26 and 27 are flowcharts illustrating exemplary methods for headerencoding and header decoding, respectively, according to anotherexemplary embodiment of this invention; and

FIG. 28 illustrates another exemplary method of passband OFDMcommunications according to this invention.

DETAIL DESCRIPTION

The exemplary embodiments of this invention will be described inrelation to OFDM communications systems, as well as protocols,techniques and methods to identify packets. However, it should beappreciated, that in general, the systems and methods of this inventionwill work equally well for other types of communications environmentsand/or protocols.

The exemplary systems and methods of this invention will also bedescribed in relation to multicarrier wired or wireless modems, such aspowerline modems, coaxial cable modems, telephone line modems, such asxDSL modems and vDSL modems, wireless modems, such as 802.11 and 802.16modems, and associated communications hardware, software andcommunications channels. However to avoid unnecessarily obscuring thepresent invention, the following description omits well-known structuresand devices that may be shown in block diagram form or are otherwisesummarized or known.

For purposes of explanation, numerous details are set forth in order toprovide a thorough understanding of the present invention. It should beappreciated however that the present invention may be practiced in avariety of ways beyond the specific details set forth herein.

Furthermore, while the exemplary embodiments illustrated herein show thevarious components of the system collocated, it is to be appreciatedthat the various components of the system can be located at distantportions of a distributed network, such as a communications networkand/or the Internet, or within a dedicated secure, unsecured, and/orencrypted system. Thus, it should be appreciated that the components ofthe system can be combined into one or more devices, such as a modem,line card, a station, a domain master, a node or collocated on aparticular node of a distributed network, such as a telecommunicationsnetwork. As will be appreciated from the following description, and forreasons of computations efficiency, the components of the systems can bearranged at any location within a distributed network without affectingthe operation thereof. For example, the various components can belocated in a domain master, a node, a domain management device, or somecombination thereof. Similarly, one or more functional portions of thissystem could be distributed between a modem and an associated computingdevice.

Furthermore, it should be appreciated that the various links, includingthe communications channels connecting the elements (not shown) can bewired or wireless links or any combination thereof, or any other knownor later developed element(s) capable of supplying and/or communicatingdata to and from the connected elements. The term module as used hereincan refer to any known or later developed hardware, software, firmware,or combination thereof, that is capable of performing the functionalityassociated with that element. The terms determine, calculate, andcompute, and variations thereof, as used herein are used interchangeablyand include any type of methodology, process, technique, mathematicaloperation or protocol. The terms transmitting modem and transmittingtransceiver as well as receiving modem and receiving transceiver arealso used interchangeably herein.

Moreover, while some of the exemplary embodiments described are directedtoward a transmitter portion of a transceiver performing certainfunctions, this disclosure is intended to include correspondingreceiver-side functionality in both the same transceiver and/or anothertransceiver and vice versa.

Certain exemplary embodiments of this invention relate to multi-carriercommunications links, such as Discrete Multi-Tone (DMT). Also, the termstransceiver and modem have the same meaning and are usedinterchangeably.

FIG. 1 illustrates an exemplary communications environment in domain 1.The communications environment includes a plurality of nodes 20-60 thatexchange packet-based OFDM communications. In FIG. 2 , an example ofmultiple devices with two profiles operating in a single domain isshown. As an example, there can be two high-profile devices (A and B),two low-profile devices (C and D), and one domain master, which is alsoa high-profile device in this exemplary embodiment. One unidirectionalcommunication link can be established between transmitter A and receiverB using a high-profile, and another unidirectional communication linkcan be made between transmitter C and receiver D using a low-profile.The domain master can manage traffic among these devices using bothprofiles. Since all devices can operate at the same time using a sharedchannel, they should be also able to identify all traffic passingthrough the domain. That is, B and D should be able to identify packetstransmitted by A and C.

FIG. 3 illustrates examples of two baseband and a passband profileswhere a low profile has a bandwidth of 50 MHz and a high profile has abandwidth of 100 MHz. In order to accommodate multiple profiles within asingle domain, all the devices should be able to identify the source anddestination of all traffic. In order to achieve this:

1. The devices operating on different profiles should be able to detecteach other's preamble.

For example, if A communicates with B, and C communicates with D in asingle domain as shown in FIG. 2 , then B should be able to detectpreambles from A and C. The same holds true for D.

2. The devices operating on different profiles may need to decode eachother's header.

For example, if A communicates with B, and C communicates with D in asingle domain as shown in FIG. 2 , then B may need to decode headersfrom A and C. The same is true for D. This is to identify the source anddestination of the packet. However, this step can optionally be omittedif the device can extract information about the profile from thereceived preamble as discussed herein.

In accordance with an exemplary embodiment of the invention is a methodto inject information regarding the profile (band plan) into a preambleso that a receiver can identify what profile the transmitter isutilizing as part of the preamble detection process. This method atleast enables the receiver to bypass the header decoding process.

FIG. 4 shows one example of the preamble structure used in variousstandards, such as IEEE 802.11. The preamble comprises S short OFDMsymbols denoted by t followed by L long OFDM symbols denoted by T. InFIGS. 4 , S=8 and L=2 as one possible example of this structure. Thereis no guard interval (G1) within short (or long) symbols, but there isone between short and long symbols. The short OFDM symbols use thesmaller number of sub-carriers (N1) with the larger tone spacing (k1),which results in a short time duration, whereas the long OFDM symbolsuse the larger number of sub-carriers (N2) with the smaller tone spacing(k2), which results in a long time duration. However, both short andlong OFDM symbols cover the same frequency band (B=N1×k1=N2×k2). Oneshould note that the same signal should be used for S short OFDM symbols(e.g., the signal becomes periodic), and the same is true for L longOFDM symbols, but short and long OFDM symbols do not necessarily havethe same signal.

In accordance with one exemplary embodiment, a technique makes use ofthe signal of the long OFDM symbols to carry information regardingprofiles. For example, if QPSK is used for all preamble symbols, thendifferent phases can be used to represent different types of profiles.For example, the preamble shown in FIG. 4 can have distinct two-sectionpreambles for 4 profiles:

-   -   Profile #1: t1(+,+), t2(+,+), . . . , t8(+,+), T1(+,+), T2(+,+)    -   Profile #2: t1(+,+), t2(+,+), . . . , t8(+,+), T1(—,+), T2(—,+)    -   Profile #3: t1(+,+), t2(+,+), . . . , t8(+,+), T1(—, —), T2(—,        —)    -   Profile #4: t1(+,+), t2(+,+), . . . , t8(+,+), T1(+, —), T2(+,        —)    -   “t1(+,+)” denotes that all tones in t1 short symbol are loaded        with (+,+).

However, the construction of the preamble as shown above and in FIG. 4is not to be considered limiting. More broadly, and in relation to FIG.5 , a portion of a packet is shown that comprises a preamble portionwith profile information as well as a header portion and a dataportion—in general, the preamble can carry information about theprofile. For example, the preamble portion of a packet can containinformation that is transmitted by a transmitter and receiver by areceiver, wherein this information indicates which profile is being usedor more generally any information about the profile being used for thetransmission or reception of the packet. The method for indicatinginformation about the packet can be, for example, based one or one ormore of the following:

the phase of subcarriers in the preamble,

the amplitude of subcarriers in the preamble,

the subcarrier spacing of subcarriers in the preamble,

the number of subcarriers in the preamble, and

the indexes of transmitted subcarriers in the preamble.

For example, FIG. 6 illustrates an exemplary transceiver 600 accordingto this invention. The transceiver 600 includes a preamble generationmodule 610, a profile embedding module 620, a profile decoding module630, controller 640, transmitter module 650, receiver module 660, packetgeneration module 670, and memory 680, as well as additional well knowncomponentry (not shown).

In operation, for packet transmission, the packet generation module 670,cooperating with the preamble generation module 610 and profileembedding module 620, include information in the preamble representativeof the profile that will be used by the transmitter module 650 for thesending of the packet. For example, as discussed above, different phasescan be used to represent different types of profiles, differentamplitudes of subcarriers can be used to represent different types ofprofiles, different subcarrier spacing of subcarriers can be used torepresent different profiles, the selection of a number of subcarrierscan be used to represent different types of profiles and the indexes oftransmitted subcarriers can be used to represent different profiles, allbeing present in the preamble.

At the receiver, upon receipt of a packet that includes profileinformation in the preamble, the profile decoding module 630, optionallyand in cooperation with controller 640 and memory 680, can decode anyone or more of the above portions of the information to determine theprofile used for transmission.

FIG. 7 illustrates an exemplary method for embedding profile informationin a packet preamble according to an exemplary embodiment of thisinvention. In particular, control begins in step S700 and continues tostep S710. In step S710, the profile to be used for packet transmissionis determined. Next, in step S720, packet generation commences. Then, instep S730, preamble generation commences. Control then continues to stepS740.

In step S740, one or more of the following techniques are used to encodeor otherwise indicate which profile will be used for transmission of thepacket. For example, in step S750, the profile information can beindicated based on the phase of the subcarriers in the preamble. In stepS752, the profile information is indicated based on the amplitude ofsubcarriers in the preamble. In step S754, the profile information isindicated based on subcarrier spacing of subcarriers in the preamble. Instep S756, the profile information is indicated based on the number ofsubcarriers in the preamble. In step S758, the profile information isindicated based on the indexes of transmitted subcarriers in thepreamble.

For example, information regarding each of these techniques can bestored in a table and, for example, memory 680. That way, both atransmitter and receiver will be able to correlate the technique usedand the information in the preamble to a specific profile.

In step S760, packet generation is completed, with transmission of thepacket occurring in step S770 and the control sequence ending in stepS780.

FIG. 8 outlines an exemplary method for profile detection according toan exemplary embodiment of this invention. In particular, control beginsin step S800 and continues to step S810. In step S810, a packet isreceived by a receiver. Steps S812-S819 illustrate one or more detectionsteps that can be performed.

In step S812, the profile used for transmission of the packet can bedetected based on the phase of the subcarriers in the preamble. In stepS814, the profile used for transmission of the packet can be detectedbased on the amplitude of subcarriers in the preamble. In step S816, theprofile used for transmission of the packet can be detected based onsubcarrier spacing of subcarriers in the preamble. In step S818, theprofile used for transmission of the packet can be detected based on thenumber of subcarriers in the preamble. In step S819, the profile usedfor transmission of the packet can be detected based on the indexes oftransmitted subcarriers in the preamble.

Then, in step S830, the profile is selected based on the detection.Control then continues to step S840.

In step S840, header decoding can optionally be bypassed, with controlcontinuing to step S850 where the control sequence ends.

FIG. 9 illustrates an exemplary method for packet transmission accordingto an exemplary embodiment of this invention. Control begins in stepS900 and continues to Step S910. In step S910, one of profile A andprofile B to be used for transmission of a packet is selected. Next, instep S920, a determination is made whether Profile A is to be used orProfile B. If Profile A is to be used control continues to step S930. Ifprofile B is to be used control jumps to step S940.

In step S930, and if Profile A is selected one or more of stepsS932-S939 are used to indicate the profile in the preamble:

-   -   Step S932—A first value for one or more phases of the        subcarriers in the preamble,    -   Step S934—A first value for one or more amplitudes of        subcarriers in the preamble,    -   Step S936—A first value for the subcarrier spacing of        subcarriers in the preamble,    -   Step S938—A first value for the number of subcarriers in the        preamble,    -   Step S939—A first set of indexes of transmitted subcarriers in        the preamble.

In step S940, and if Profile B is selected, one or more of stepsS942-S949 are used to indicate the profile in the preamble:

-   -   Step S942—A second value for one or more phases of the        subcarriers in the preamble,    -   Step S944—A second value for one or more amplitudes of        subcarriers in the preamble,    -   Step S946—A second value for the subcarrier spacing of        subcarriers in the preamble,    -   Step S948—A second value for the number of subcarriers in the        preamble,    -   Step S949—A second set of indexes of transmitted subcarriers in        the preamble.

In step S950 packet generation is completed with the packet beingtransmitted in step S960. Control then continues to step S970 where thecontrol sequence ends.

FIG. 10 illustrates an exemplary method for packet reception accordingto an exemplary embodiment of this invention. Control begins in stepS1000 and continues to Step S1010. In step S1010, a packet is receivedthat was transmitted with either Profile A or Profile B. Next, in stepS1020, the values in one or more of steps S1022-S1029 are detected:

-   -   Step S1022—A first or second value for one or more phases of the        subcarriers in the preamble,    -   Step S1024—A first or second value for one or more amplitudes of        subcarriers in the preamble,    -   Step S1026—A first or second for the subcarrier spacing of        subcarriers in the preamble,    -   Step S1027—A first or second for the number of subcarriers in        the preamble,    -   Step S1029—A first or second set of indexes of transmitted        subcarriers in the preamble.

Control then continues to step S1030.

In step S1030, the profile used for transmission is determined based onthe detection above, wherein the first values correspond to Profile Aand the second values correspond to Profile B. Control then continues tostep S1040 where the control sequence ends.

Header Identification

Another exemplary embodiment of this invention is directed towardsmethods that allow common sub-carrier indexing for all profiles so that,for example, a high-profile device can decode a portion of sub-carriersof a low-profile device, and a low-profile device can decode a portionof sub-carriers of a high-profile device.

For example, and again with reference to FIG. 2 , C should be able todecode a portion of sub-carriers from packets that A transmits, and viceversa. This will be a prerequisite to decode each other's headerinformation to identify the source and destination of data packets. Toneindexing determines the order in which a plurality of bits are mapped to(or loaded on) subcarriers. Therefore, common tone indexing is importantfor communication between devices using different profiles.

The following is one example of tone indexing based on a conventionalIDFT definition.

The sub-carriers shall be indexed from i=0 to i=N−1. The sub-carrierswith indexes from i=0 to i=N/2−1 shall be centered at frequenciesF(i)=Fuc+Fus+i×Fsc, while those with indexes from i=N/2 to i=N−1 shallbe centered at frequencies F(i)=Fuc+Fus−(N−i)×Fsc. In basebandtransmission, Fuc=0, while in passband transmission, Fc=Fuc+Fus.

Based on the definition described above, the sub-carrier indexing isshown in FIG. 11 . Note that sub-carrier indices in the overlappedfrequency range do not match in two different profiles, which makescommunication between these two profiles impossible.

FIG. 12 shows one exemplary embodiment of alternate sub-carrier indexingwhich makes all sub-carrier indices in the overlapped frequency rangematch in these two profiles according to this invention. There can bemany different ways to achieve the same goal. Of note is thatsub-carrier indices in the common overlapped frequency region shouldmatch for all different profiles.

Let N=K×M where K is the number of frequency segments, which representsthe maximum number of profiles, and M is the number of sub-carrierswithin each segment. In the example in FIG. 5 , N=512, K=2, M=256.

Baseband:F(i)=i×Fsc if i=0, . . . ,N−1

Passband:

For each segment k=0, 1, . . . , K−1,F(i)=Fc+(i−k×M/2)×Fsc if i=k×M, . . . ,k×M+M/2−1F(i)=Fc−(k×3M/2+M−i)×Fsc if i=k×M+M/2, . . . ,(k+1)×M−1

where Fc=Fuc+Fus and j=i−k×M.

FIG. 13 shows another example of passband sub-carrier indexing whichmakes all sub-carrier indices in the overlapped frequency range match inmultiple profiles. This approach has an exemplary advantage over theprevious approach because the index increases monotonically from thecenter frequency to the outward direction (both left and right) whichmakes the scheme more scalable.

Passband:F(i)=Fc+(i/2)×Fsc if i=0,2,4, . . . ,N−2.F(i)=Fc−(i+1)/2×Fsc if i=1,3,5, . . . ,N−1.

where Fc=Fuc+Fus.

FIG. 14 illustrates an exemplary transceiver 1200 that indexessubcarriers in OFDM communications according to an exemplary embodimentof this invention. Transceiver 1200 includes a subcarrier managementmodule 1210, a header decoding module 1220, a transmitter module 1250, areceiver module 1260, controller 1230 and memory 1240, as well as otherwell known componentry (not shown).

In operation, the subcarrier management module 1210, cooperating withcontroller 1230 and memory 1240, a plurality of bits to subcarriers in aspecific order, for example, as illustrated in relation to FIG. 13 .

At any receiver, and in conjunction with a header decoding module, theheader information can be decoded to identify the source and destinationof data packets.

For example, one method of indexing subcarriers for transmission inpassband OFDM communications is outlined in FIG. 15 .

Specifically, control begins in step S1500 and continues to step S1510.In step S1510, a frequency F(i_(even)) is assigned to subcarriers witheven indices in accordance with:F(i _(even))=Fc+(i/2)×Fsc;

Next, in step S1520, a frequency F(i_(odd)) is assigned to subcarrierswith odd indices in accordance with:F(i _(odd))=Fc−(i+1)/2×Fsc;

where Fc=Fuc+Fus, and wherein i is an index, Fuc is a first frequencyvalue and Fus is a second frequency value and Fsc is third frequencyvalue.

Then, in step S1530, a plurality of bits are loaded onto the subcarriersin an order determined by the indices. Control then continues to stepS1540 where the control sequence ends.

One exemplary method of indexing subcarriers for reception in passbandOFDM communications is illustrated in FIG. 16 . Specifically, controlbegins in step S1600 and continues to Step S1610. In step S1610, afrequency F(i_(even)) is assigned to subcarriers with even indices inaccordance with:F(i _(even))=Fc+(i/2)×Fsc;

Next, in step S1620, a frequency F(i_(odd)) is assigned to subcarrierswith odd indices in accordance with:F(i _(odd))=Fc−(i+1)/2×Fsc; where Fc=Fuc+Fus, and

wherein i is an index, Fuc is a first frequency value and Fus is asecond frequency value and Fsc is third frequency value.

Then, in step S1630, a plurality of bits from the subcarriers isdemodulated in an order determined by the indices. Control thencontinues to step S1640 where the control sequence ends.

Another exemplary method of indexing subcarriers for transmission inpassband OFDM communications is outlined in FIG. 17 .

Specifically, control begins in step S1700 and continues to step S1710.In step S1710, bits are modulated and/or encoded and/or loaded ontosubcarriers in an order specified by a predetermined indexing technique.The predetermined indexing technique specifies that:

a frequency F(i_(even)) is assigned to subcarriers with even indices inaccordance with:F(i _(even))=Fc+(½)×Fsc; and,a frequency F(i_(odd)) is assigned to subcarriers with odd indices inaccordance with:F(i _(odd))=Fc−(i+1)/2×Fsc;where Fc=Fuc+Fus, and wherein i is an index, Fuc is a first frequencyvalue and Fus is a second frequency value and Fsc is third frequencyvalue. Control then continues to step S1720 where the control sequenceends.

Another exemplary method of indexing subcarriers for reception inpassband OFDM communications is illustrated in FIG. 18 . Specifically,control begins in step S1800 and continues to Step S1810.

In step S1810, a plurality of bits from the subcarriers is demodulatedand/or decoded in an order specified by a predetermined indexingtechnique. The predetermined indexing technique specifies that:

a frequency F(i_(even)) is assigned to subcarriers with even indices inaccordance with:F(i _(even))=Fc+(½)×Fsc;

a frequency F(i_(odd)) is assigned to subcarriers with odd indices inaccordance with:F(i _(odd))=Fc−(i+1)/2×Fsc; where Fc=Fuc+Fus,

wherein i is an index, Fuc is a first frequency value and Fus is asecond frequency value and Fsc is third frequency value.

Control then continues to step S1820 where the control sequence ends.

Header Decodability Enhancements

Yet another exemplary embodiment of this invention is directed towardmethods to enhance decodability of the header via sub-carrierredistribution. These techniques can be used independently from or inconjunction with the aforementioned methods. However, these methods canalso be used as a complementary means to, for example, mitigate theside-effect of passband sub-carrier indexing shown in FIG. 13 as well asto provide the receiver a better chance to estimate the channel.

As illustrated in FIG. 4 , the conventional preamble design uses thelarger sub-carrier spacing than the header or the payload in order toreduce the overhead (i.e., k1, k2>1). This in result makes the receiverdesign complicated because the channel response for sub-carriers whichdo not carry preamble signals (“non-preamble tones”) can be onlyobtained from neighboring sub-carriers which carry preamble signals(“preamble tones”). Therefore, the SNR for non-preamble tones is lowerthan the SNR for preamble tones. Based on the preamble, the channelshould be accurately estimated for all sub-carriers since the header andthe payload utilize all sub-carriers to carry information.

The conventional header design often contains multiple copies of thefixed (and often small) number of header information bits (header block)within a given bandwidth in order to increase frequency diversity. Thisis usually the case because the number of the header information bits isrelatively small, and header information is often carried over one ormore OFDM symbols which have much higher capacity. It is shown in FIG.19 that low profile and high profile contains the same amount of headerinformation, but different number of copies of the header blocks. Thatis, a low-profile device decoding only a portion of sub-carriers stillcan decode the full header information even though with decreasedfrequency diversity.

The indexing rules shown in FIGS. 12 and 13 are used for baseband andpassband profiles, respectively.

If a certain portion of header information always ends up being loadedon non-preamble tones, then it can degrade decodability of the entireheader. In order to avoid such a case, one exemplary embodiment providesvarious sub-carrier redistribution methods for the header as a singleheader block is repeated multiple times over the entire bandwidth. Thiswill guarantee that header information is evenly distributed betweenpreamble and non-preamble tones.

Conventional Header Repetition Scheme

Assume that the header information bits will be encoded with r-rate FECand modulated with M-bit constellation. The encoder output will berepeated multiple times to fill out one or more OFDM symbols and mappedto sub-carriers. That is, KH header information bits are encoded to NHFEC codeword (NH=1/r×KH), and the 1st through Mth bits are loaded on thefirst tone, and the (M+1)th through (2M)th bits are loaded on the secondtone, and so on. The tone block which corresponds to a single set of theheader information will be referred to as the “header tone block” andthe tone index within each block as the “header tone index.”

For the purpose of illustration, the following parameters are used:

-   -   Maximum number of subcarriers: N=2048.    -   Preamble structure: two-section preamble as described in §        4.3.2.    -   Tone spacing for preamble: Fsc_preamble=8×Fsc.    -   For baseband bandplan, preamble tones are located at        (k×Fsc_preamble) where k=4, 12, 20, . . . , 2044 (total of 256        tones).    -   For RF bandplan, preamble tones are located at        (Fc+k×Fsc_preamble) where k=±4, ±12, ±20, . . . , ±1020 (total        of 256 tones).    -   Header information block size: KH=168.    -   ½-rate FEC with QPSK modulation (r=½, M=2).    -   FEC Encoded Header information block size=168×2=336    -   Tone spacing for the header: Fsc_header=FSC.    -   One OFDM symbol for the header.    -   Baseband sub-carrier indexing (FIG. 10 ) and Passband        sub-carrier indexing (FIG. 11 ).

For this case, a single set of header information (168 bits) will be FECencoded with the r=½ code resulting in an FEC encoded header informationblock size of 336 at the output of the FEC encoder. These 336 FECencoded bits (or FEC bits) will be mapped to 168 sub-carriers with eachbit modulating 2 bits (M=2). Since there are a total of 2048 tones in agiven bandwidth, header information will be repeated a little more than12 times over the entire bandwidth in one OFDM symbol provided that allsub-carriers are active. For all sub-carriers i=0, . . . , 2047, eachheader tone block would be mapped as {0, 1, 2, . . . , 167}, {0, 1, 2, .. . , 167}, . . . .

FIG. 20 illustrates how each sub-carrier in the header block is mappedto the actual frequency in the conventional header repetition scheme.

There are at least two exemplary problems with this repetition scheme:

1. In the baseband case, it can be easily seen that the preamble tonesalways carry a certain portion of header information. That is, 4th,12th, . . . , 164th header indices, corresponding to FEC bit pairs(8,9), (24,25 and (228,229) respectively, are always carried on preambletones, whereas the rest of the FEC bits are always carried onnon-preamble tones. A similar thing happens in the passband case. Thisis because the number of header information bits is a multiple of 8(ratio of the number of non-preamble tones over the number of preambletones in a given bandwidth). This is not desirable because theperformance of the header can be limited by information carried onnon-preamble tones.

2. In the passband case, the header information corresponding to evenheader tone indices always end up being on the right-half of the centerfrequency, and likewise odd header tone indices on the left-half. Thisis a side effect of alternating odd-even tone indexing scheme.

Enhanced Header Repetition Schemes

Circular Shifting Sub-Carriers within Each Header Block

One exemplary technique is to change tone mapping as a copy of theheader block is repeated over the entire bandwidth. For example, as eachblock of header information is repeated, a circular shift can beperformed on the FEC codeword bits or the header indices. For instance,each header tone block can be mapped as such that the header indexfollows the following pattern: {0, 1, . . . , 166, 167}, {1, 2, . . . ,167, 0}, {2, 3, . . . , 167, 0, 1}, . . . for all sub-carriers i=0, . .. , 2047. Equivalently, the FEC encoder output can be circular-shiftedby 2 bits after each consecutive codeword is concatenated together. Notethat the outcome of this approach is similar to that of the previousapproach.

FIG. 22 shows how pairs of FEC encoded bits are mapped to subcarriers.Note that different encoded bits are loaded on preamble tones for thesecond copy of header block. More specifically, a first set of bits fromthe first copy of the FEC codeword is modulated on a first set ofpreamble subcarriers and a second different set of bits from the secondcopy of the FEC codeword is modulated on a second set of preamblesubcarriers. This is a result of the cyclically shifting of bits of FECEncoded Header information block.

Also for the passband case, even and odd indices are evenly allocatedaround the center frequency.

FIG. 23 illustrates an example of a transceiver 2300 capable ofperforming the enhanced header techniques disclosed herein. Thetransceiver 2300 comprises an FEC Encoder/Decoder 2310, a header encoder2320, a header decoder 2330, controller 2340, memory 2350, a transmittermodule 2360, and a receiver module 2370 and amodulator/encoder/demodulator/decoder module 2380, in addition to otherwell known components (not shown).

In one exemplary transmitter embodiment, a header is encoded formulticarrier communications in accordance with the following technique.Specifically, the FEC encoder 2310 encodes a plurality of bits using aforward error correction code and generates an FEC codeword comprising aplurality of encoded bits. The header encoder 2330 concatenates at leasta first copy of the FEC codeword with at least a second copy of the FECcodeword and wherein the concatenating comprises cyclically shifting bytwo bits the second concatenated copy of the FEC codeword relative tothe first concatenated copy of the FEC codeword. The modulating module2380 then modulates first and second copies of the FEC codewords on atleast one OFDM symbol for transmission by the transmitter module 2360.

In another exemplary transmitter embodiment, a header is encoded formulticarrier communications in accordance with the following technique.Specifically, the FEC encoder 2310 encodes a plurality of bits using aforward error correction code and generates an FEC codeword comprising aplurality of encoded bits. The header encoder 2320 concatenates aplurality of copies of the FEC codeword wherein the concatenatingcomprises cyclically shifting by two bits at least one concatenated copyof the FEC codeword relative to a previous concatenated copy of the FECcodeword. The modulating module 2380 then modulates the plurality ofcopies of the FEC codewords on at least one OFDM symbol for transmissionby the transmitter module 2360.

In one exemplary receiver embodiment, the header is decoded inaccordance with the following exemplary technique. The transceiver 2300receives, with the cooperation of the receiver module 2370, a pluralityof encoded bits comprising at least two concatenated copies of an FECcodeword. The FEC decoder 2310 decodes a first copy of the FEC codewordfrom the plurality of encoded bits to generate a first plurality ofdecoded bits. The FEC decoder 2310 decodes a second copy of the FECcodeword from the plurality of encoded bits to generate a secondplurality of decoded bits, wherein the decoding comprises cyclicallyshifting by two bits the second copy of the FEC codeword relative to thefirst copy of the FEC codeword.

In another exemplary receiver embodiment, the header is decoded inaccordance with the following exemplary technique. The transceiver 2300receives, with the cooperation of the receiver module 2370, a pluralityof encoded bits comprising a plurality of concatenated copies of an FECcodeword. The FEC decoder 2310 decodes at least two of the plurality ofthe concatenated copies of the FEC codeword wherein the decodingcomprises cyclically shifting by two bits at least one concatenated copyof the FEC codeword relative to a previous concatenated copy of the FECcodeword.

FIGS. 24 and 25 illustrate exemplary methods for header encoding andheader decoding, respectively, according to an exemplary embodiment ofthis invention.

In FIG. 24 , control for header encoding in multicarrier communicationscommences in step S2400 and continues to step S2410. In step S2410, aplurality of bits are encoded using a forward error correction code.Next, in step S2420, an FEC codeword is generated comprising a pluralityof encoded bits. Then, in step S2430, a first copy of the FEC codewordis concatenated with a second copy of the FEC codeword wherein theconcatenating comprises cyclically shifting by two bits the secondconcatenated copy of the FEC codeword relative to the first concatenatedcopy of the FEC codeword. Next, in step S2440, the encoded bits of thefirst and second copies of the FEC codewords are modulated on at leastone OFDM symbol. Control then continues to step S2450 where the controlsequence ends.

In FIG. 25 , control begins in step S2500 and continues to step S2510.In step S2510, a plurality of encoded bits comprising at least twoconcatenated copies of an FEC codeword is received. Next, in step S2520,a first copy of the FEC codeword is decoded to generate a firstplurality of decoded bits. Next, in step S2530, a second copy of the FECcodeword is decoded to generate a second plurality of decoded bits,wherein the decoding comprises cyclically shifting by two bits thesecond copy of the FEC codeword relative to the first copy of the FECcodeword. Control then continues to step S2540 where the controlsequence ends.

FIGS. 26 and 27 illustrate exemplary methods for header encoding andheader decoding, respectively, according to another exemplary embodimentof this invention.

In FIG. 26 , control for header encoding in multicarrier communicationscommences in step S2600 and continues to step S2610. In step S2610, aplurality of bits are encoded using a forward error correction code.Next, in step S2620, an FEC codeword is generated comprising a pluralityof encoded bits. Then, in step S2630, a plurality of copies of the FECcodeword are concatenated wherein the concatenating comprises cyclicallyshifting by two bits at least one concatenated copy of the FEC codewordrelative to a previous concatenated copy of the FEC codeword. Next, instep S2640, the plurality of copies of the FEC codewords are modulatedon at least one OFDM symbol. Control then continues to step S2450 wherethe control sequence ends.

In FIG. 27 , control begins in step S2700 and continues to step S2710.In step S2710, a plurality of encoded bits comprising a plurality ofcopies of an FEC codeword is received. Next, in step S2720, at least twoof the plurality of copies of the FEC codeword are decoded to generate afirst plurality of decoded bits wherein the decoding comprisescyclically shifting by two bits at least one concatenated copy of theFEC codeword relative to a previous concatenated copy of the FECcodeword. Control then continues to step S2730 where the controlsequence ends.

FIG. 28 illustrates another exemplary method of passband communicationsaccording to this invention. Control begins in step S2800 and continuesto step S2810. In step S2810, a sequence of bits are modulated and/orencoded and/or loaded onto subcarriers in a predetermined order, whereinthe predetermined order includes:

first modulating and/or encoding and/or loading bits from the sequenceof bits to a subcarrier with a frequency value that is greater than acenter frequency value, and

second modulating and/or encoding and/or loading bits from the sequenceof bits to a subcarrier with a frequency value that is less than thecenter frequency value. Alternatively, or in addition, the sequence ofbits are modulated and/or encoded and/or loaded onto subcarriers in apredetermined order, wherein the predetermined order comprisesalternating between modulating and/or encoding and/or loading bits ontosubcarrier frequencies that are greater than a center frequency andmodulating and/or encoding and/or loading bits onto subcarrierfrequencies that are less than the center frequency. The subcarrierfrequency value can be greater than the center frequency value isFc+n×Fsc and the subcarrier frequency value that is less than the centerfrequency is Fc−n×Fsc, wherein Fc is the value of center frequency in Hzand, Fsc is a value for subcarrier spacing in Hz, and n is an integer.Next, in step S2820, the sequence of bits is transmitted from a firsttransceiver. Control then continues to step S2830.

In step S2830, the sequence of bits are received at a secondtransceiver. Next, in step S2840, the sequence of bits are decodedand/or demodulated from subcarriers in an predetermined order, whereinthe predetermined order comprises first decoding and/or demodulatingbits from the sequence of bits from a subcarrier with a frequency valuethat is greater than a center frequency value and second decoding and/ordemodulating bits from the sequence bits from a subcarrier with afrequency value that is less than the center frequency value. Thesubcarrier frequency value that is greater than the center frequencyvalue is Fc+n×Fsc, and wherein the subcarrier frequency value that isless than the center frequency is Fc−n×Fsc, wherein Fc is the value ofcenter frequency in Hz and, Fsc is a value for subcarrier spacing in Hz,and n is an integer. Alternatively or in addition, the sequence of bitscan be decoded and/or demodulated from subcarriers in a predeterminedorder, wherein the predetermined order comprises alternating betweendecoding and/or demodulating bits from subcarrier frequencies that aregreater than a center frequency and decoding and/or demodulating bitsfrom subcarrier frequencies that are less than the center frequency.Control then continues to step S2850 where the control sequence ends.

The above-described methods and systems and can be implemented in asoftware module, a software and/or hardware testing module, atelecommunications test device, a DSL modem, an ADSL modem, an xDSLmodem, a VDSL modem, a linecard, a G.hn transceiver, a MOCA transceiver,a Homeplug transceiver, a powerline modem, a wired or wireless modem,test equipment, a multicarrier transceiver, a wired and/or wirelesswide/local area network system, a satellite communication system,network-based communication systems, such as an IP, Ethernet or ATMsystem, a modem equipped with diagnostic capabilities, or the like, oron a separate programmed general purpose computer having acommunications device or in conjunction with any of the followingcommunications protocols: CDSL, ADSL2, ADSL2+, VDSL1, VDSL2, HDSL, DSLLite, IDSL, RADSL, SDSL, UDSL, MOCA, G.hn, Homeplug® or the like.

Additionally, the systems, methods and protocols of this invention canbe implemented on a special purpose computer, a programmedmicroprocessor or microcontroller and peripheral integrated circuitelement(s), an ASIC or other integrated circuit, a digital signalprocessor, a flashable device, a hard-wired electronic or logic circuitsuch as discrete element circuit, a programmable logic device such asPLD, PLA, FPGA, PAL, a modem, a transmitter/receiver, any comparablemeans, or the like. In general, any device capable of implementing astate machine that is in turn capable of implementing the methodologyillustrated herein can be used to implement the various communicationmethods, protocols and techniques according to this invention. While thesystems and means disclosed herein are described in relation to variousfunctions that are performed, it is to be appreciated that the systemsand means may not always perform all of the various functions, but arecapable of performing one or more of the disclosed functions.

Furthermore, the disclosed methods may be readily implemented insoftware using object or object-oriented software developmentenvironments that provide portable source code that can be used on avariety of computer or workstation platforms. Alternatively, thedisclosed system may be implemented partially or fully in hardware usingstandard logic circuits or VLSI design. Whether software or hardware isused to implement the systems in accordance with this invention isdependent on the speed and/or efficiency requirements of the system, theparticular function, and the particular software or hardware systems ormicroprocessor or microcomputer systems being utilized. Thecommunication systems, methods and protocols illustrated herein can bereadily implemented in hardware and/or software using any known or laterdeveloped systems or structures, devices and/or software by those ofordinary skill in the applicable art from the functional descriptionprovided herein and with a general basic knowledge of the computer andtelecommunications arts.

Moreover, the disclosed methods may be readily implemented in softwarethat can be stored on a computer-readable medium, executed on programmedgeneral-purpose computer with the cooperation of a controller andmemory, a special purpose computer, a microprocessor, or the like. Inthese instances, the systems and methods of this invention can beimplemented as program embedded on personal computer such as an applet,JAVA® or CGI script, as a resource residing on a server or computerworkstation, as a routine embedded in a dedicated communication systemor system component, or the like. The system can also be implemented byphysically incorporating the system and/or method into a software and/orhardware system, such as the hardware and software systems ofcommunication device.

While the invention is described in terms of exemplary embodiments, itshould be appreciated that individual aspects of the invention could beseparately claimed and one or more of the features of the variousembodiments can be combined.

While the systems and means disclosed herein are described in relationto various functions that are performed, it is to be appreciated thatthe systems and means may not always perform all of the variousfunctions, but are capable of performing one or more of the disclosedfunctions.

While the exemplary embodiments illustrated herein disclose the variouscomponents as collocated, it is to be appreciated that the variouscomponents of the system can be located at distant portions of adistributed network, such as a telecommunications network and/or theInternet or within a dedicated communications network. Thus, it shouldbe appreciated that the components of the system can be combined intoone or more devices or collocated on a particular node of a distributednetwork, such as a telecommunications network. As will be appreciatedfrom the following description, and for reasons of computationalefficiency, the components of the communications network can be arrangedat any location within the distributed network without affecting theoperation of the system.

It is therefore apparent that there has been provided, in accordancewith the present invention, systems and methods for PSD management.While this invention has been described in conjunction with a number ofembodiments, it is evident that many alternatives, modifications andvariations would be or are apparent to those of ordinary skill in theapplicable arts. Accordingly, it is intended to embrace all suchalternatives, modifications, equivalents and variations that are withinthe spirit and scope of this invention.

The invention claimed is:
 1. A wireless transceiver comprising atransmitter portion and a receiver portion, the transmitter portion ofthe transceiver comprising: an FEC (Forward Error Correction) encodercapable of encoding a plurality of bits using a forward error correctioncode and capable of generating an FEC codeword comprising a plurality ofencoded bits; and a header encoder capable of concatenating a first copyof the FEC codeword with a second copy of the FEC codeword, wherein theconcatenating comprises cyclically shifting by two bits the secondconcatenated copy of the FEC codeword relative to the first concatenatedcopy of the FEC codeword, and the receiver portion of the transceivercomprising: a header decoder capable of receiving a plurality of encodedbits comprising at least two concatenated copies of a received FECcodeword; and an FEC decoder capable of decoding a first copy of thereceived FEC codeword to generate a first plurality of decoded bits, andcapable of decoding a second copy of the received FEC codeword togenerate a second plurality of decoded bits, wherein the decodingcomprises cyclically shifting by two bits the second copy of thereceived FEC codeword relative to the first copy of the received FECcodeword.
 2. The wireless transceiver of claim 1, wherein thetransmitter portion further comprising a modulator capable of modulatethe encoded bits of the first and second copies of the FEC codewords onat least one OFDM symbol.
 3. The wireless transceiver of claim 2,wherein the receiver portion further comprising a demodulator capable ofdemodulating the encoded bits of the first and second copies of thereceived FEC codewords from at least one received OFDM symbol.
 4. Amethod in a wireless transceiver comprising a transmitter portion and areceiver portion, the method in the transmitter portion of thetransceiver comprising: encoding, by an FEC encoder, a plurality of bitsusing a forward error correction code; generating, by the FEC encoder,an FEC codeword comprising a plurality of encoded bits; andconcatenating, by a header encoder, a first copy of the FEC codewordwith a second copy of the FEC codeword, wherein the concatenatingcomprises cyclically shifting by two bits the second concatenated copyof the FEC codeword relative to the first concatenated copy of the FECcodeword, and the method in the receiver portion of the transceivercomprising: receiving, by a header decoder, a plurality of encoded bitscomprising at least two concatenated copies of a received FEC (ForwardError Correction) codeword; and decoding, by an FEC decoder, a firstcopy of the received FEC codeword to generate a first plurality ofdecoded bits, and decoding a second copy of the received FEC codeword togenerate a second plurality of decoded bits, wherein the decodingcomprises cyclically shifting by two bits the second copy of thereceived FEC codeword relative to the first copy of the received FECcodeword.
 5. The method of claim 4, wherein the method of thetransmitter portion further comprises modulating, by a modulator, theencoded bits of the first and second copies of the FEC codewords on atleast one OFDM symbol.
 6. The method of claim 4, wherein the method ofthe receiver portion further comprises demodulating, by a demodulator,the encoded bits of the first and second copies of the received FECcodewords from at least one received OFDM symbol.
 7. A non-transitorycomputer-readable information storage media having stored thereoninstructions, that when executed by one or more processors, cause to beperformed in a wireless transceiver comprising a transmitter portion anda receiver portion, and a method in the transmitter portion of thetransceiver comprising: encoding, by an FEC encoder, a plurality of bitsusing a forward error correction code; generating, by the FEC encoder,an FEC codeword comprising a plurality of encoded bits; andconcatenating, by a header encoder, a first copy of the FEC codewordwith a second copy of the FEC codeword, wherein the concatenatingcomprises cyclically shifting by two bits the second concatenated copyof the FEC codeword relative to the first concatenated copy of the FECcodeword, and a method in the receiver portion of the transceivercomprising: receiving, by a header decoder, a plurality of encoded bitscomprising at least two concatenated copies of a received FEC (ForwardError Correction) codeword; and decoding, by an FEC decoder, a firstcopy of the received FEC codeword to generate a first plurality ofdecoded bits, and decoding a second copy of the received FEC codeword togenerate a second plurality of decoded bits, wherein the decodingcomprises cyclically shifting by two bits the second copy of thereceived FEC codeword relative to the first copy of the received FECcodeword.
 8. The media of claim 7, wherein the method of the transmitterportion further comprises modulating, by a modulator, the encoded bitsof the first and second copies of the FEC codewords on at least one OFDMsymbol.
 9. The media of claim 7, wherein the method of the receiverportion further comprises demodulating, by a demodulator, the encodedbits of the first and second copies of the received FEC codewords fromat least one received OFDM symbol.